With near flawless precision, a spacecraft slipped into orbit around the asteroid Eros, becoming the first manmade satellite of an asteroid. The craft now starts a year-long close-up study of the potato-shaped space rock, hoping to determine its origins and help scientists mount a strategy to protect Earth from boulders from outer space.

A short rocket firing on Monday changed the orbital path of the Near Earth Asteroid Rendezvous (NEAR) spacecraft to allow it to settle into an orbit of Eros. The manoeuvre successfully completed a four-year odyssey that included a failed attempt to rendezvous with the rock last year.

"The NEAR spacecraft is in orbit around the asteroid Eros," mission director Robert Farquhar announced minutes after engineers received a signal that a rocket had fired with an error of less than 1 percent.

"We are very happy and very excited," said Farquhar.

Automatic instructions stored aboard the craft triggered the rocket firing at 10:33am (15:33 GMT) and engineers waited tensely in Mission Control for almost 20 minutes before success was confirmed. They broke into applause when it was clear that NEAR had reached its target.

Farquhar said the rocket firing aimed for an Eros orbit of about 200 miles by 299 miles (320 by 480 kilometers) and the craft hit that target within 30 to 40 miles (50 to 65 kilometres). This is considered excellent "shooting" for such a small object that is 160 million miles (260 million kilometres) away.

Eros is so far out that a radio signal takes 14.5 minutes to reach Earth. Project scientist Andrew Cheng said the success thrilled scientists eager to get an unprecedented close-up view of an asteroid and gather data with five different instruments.

"Today may be Valentine's Day for most people, but its Christmas Eve for me and all the presents are piled about, waiting to be opened," said Cheng.

NEAR will spend a year orbiting Eros, dropping in stages to lower orbits. It will gather basic research that one day may help humans defend the Earth against a "killer asteroid" like the one thought to have wiped out the dinosaurs 65 million years ago.

"Understanding the physical characteristics of asteroids will be very important if we are ever called on to deflect one coming at the Earth," said Nasa's Carl Pilcher.

Although Eros itself poses no threat to Earth, a dangerous asteroid could be discovered at any time, Cheng said.

"This is something humanity needs to be concerned about," said Cheng. "It has happened before on Earth." In 1989, an asteroid a quarter-mile wide missed the Earth by just 400,000 miles (640,000 kilometres). Astronomers said the space rock and Earth passed the same point in space just six hours apart.

Scientists also hope to learn more about the formation of planets, such as Earth, since Eros may be leftover material from the creation of the solar system.

Eros is 21 miles long and eight miles wide; by comparison, New York's Manhattan Island is about 13.5 miles long and 2.5 miles wide.

The $224 million NEAR mission was launched February 17, 1996. The craft was supposed to have gone into an orbit of Eros in January 1999, but it malfunctioned after a rocket firing - intended to aim the spacecraft at the asteroid - shut down automatically.

NEAR sped past Eros and then went into wild gyrations, cutting its contact with Earth. When engineers again captured the NEAR signal, they found the craft had burned about 20 percent of its fuel.

Engineers let NEAR circle the sun for a year, guiding it with fuel-saving small rocket firings toward a Valentine's Day rendezvous with Eros. This time the spacecraft performed without a problem.

"Another perfect day," Farquhar announced.

During its mission, as NEAR descends closer and closer to Eros, the craft late in the year may actually brush the asteroid's surface with a solar panel and photograph the mark it leaves.

NEAR was built under Nasa contract at John Hopkins University Applied Physics Laboratory. Scientists from Cornell University and the Jet Propulsion Laboratory also participated.

NEAR has five instruments, including a camera that will map and probe every surface inch of Eros. The instruments will measure density, chemical composition and magnetic fields. The camera will send photos in different spectra, in black and white and colour.